Reinforced polyvinyl alcohol hydrogels containing uniformly dispersed crystalline fibrils and method for preparing same

ABSTRACT

This invention provides reinforced composite structures consisting essentially of a matrix and a dispersed phase, where the matrix is a hydrogel containing water and a solubilized vinyl alcohol homopolymer, the dispersed phase contains a plurality of uniformly distributed acicular regions consisting of fibrils formed from a highly oriented crystalline vinyl alcohol polymer, where the diameter of the said fibrils is less than 1 millimeter, the aspect ratio of the fibrils is from 2:1 to 1000:1, and where the composite is characterized by a gradual transition in the degree of crystallinity at the interfaces between the matrix and the fibrils.

This is a divisional of copending application Ser. No. 07/990,390 filedon Dec. 14, 1992; now U.S. Pat. No. 5,336,651.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polyvinyl alcohol hydrogels. Moreparticularly, this invention relates to polyvinyl alcohol hydrogelscontaining uniformly dispersed and highly oriented crystalline regionsthat substantially improve the physical properties of the hydrogel.

2. Background of the Invention

The chemical and physical properties of polyvinyl alcohol hydrogels makethem useful materials for a variety of end use applications, includingfiltration and ion exchange membranes, bio-compatible films, fibers andother shaped articles, contact lenses, devices for the controlleddelivery of materials, coatings and filler materials.

The formation and physical properties of polyvinyl alcohol hydrogels arediscussed in a review article by Hyon in the journal "Kobunshi Kako"39(6) 304-10 (1990) [CAl14(16):144304x].

Gels, fibers and other products prepared from solutions of polyvinylalcohol in water or a mixture of water and a water-miscible organicsolvent such as methanol, propanol, dimethyl sulfoxide andN,N-dimethylformamide are well known from numerous patents and otherpublications. Typical of recently issued patents describing gels formedfrom aqueous solutions of polyvinyl alcohol is U.S. Pat. No. 4,663,358,which issued to Hyon et al on May 5, 1987 and U.S. Pat. No. 4,851,168,which issued to D. Graiver et al. on Jul. 25, 1989. These patents teachcooling to below room temperature a solution of polyvinyl alcohol in amixture of water and a water-miscible organic solvent such as alcoholscontaining from 1 to 4 carbon atoms, glycols and dimethyl sulfoxide.Mixtures of water and dimethyl sulfoxide are preferred, and the watercan constitute from 10 to 90 weight percent of the solvent mixture. Thegel formed by cooling the solution Is then immersed in flowing water toremove the organic solvent.

Hydrogels prepared as described in the Hyon and Graiver et al. patentsare transparent whereas gels formed using a solution of polyvinylalcohol in either water or dimethyl suifoxide as the only solvent areopaque.

For some end uses such as biomedical applications it is desirable tomaximize the tensile strength and other physical properties of polyvinylalcohol hydrogels. This is particularly true for hydrogels containingmore than about 40 weight percent of water.

Techniques that have been used to improve the physical properties ofpolyvinyl alcohol hydrogels typically involve crosslinking by radiationor chemical means, increasing the crystallinity of the polymer and/oradding reinforcing agents. The improvements in tensile propertiesachieved by crosslinking are typically less than desirable, and areaccompanied by reduced swelling in water, which is desirable for someapplications, and insolubility of the polymer in water and organicsolvents such as hot water and dimethyl suifoxide that dissolvenon-crosslinked polymers.

The aforementioned U.S. Pat. No. 4,851,168, issued to D. Graiver et al.and Japanese patent publication no. 1/257,026, published on Oct. 13,1989 teaches preparing high strength fibers and films from polyvinylalcohol (PVA) hydrogels by spinning or extruding solutions of PVA withdegrees of polymerization greater than 1500 and drawing the resultantproducts using draw ratios greater than 10. The Japanese patentpublication teaches shrinking the resultant fiber or film in a waterbath at a temperature at least 5 degrees below but not more than 50degrees below the temperature at which the PVA will dissolve in thebath.

Hyon et al. in the journal "Kobunshi Ronbunshu" 46 (11) 673-80 (1989)[CA112(12):99763s] teach freezing a concentrated aqueous solution of PVAfollowed by slow crystallization of the frozen polymer above itsfreezing point to produce a semi-crystalline polymer with a microporousstructure.

The use of repeated freeze-thaw cycles to increase the strength andrigidity of PVA hydrogels intended to control the release ofpharmaceuticals and serum albumin is taught by N. Peppas et al. [J.Controlled release, 18(2), 95-100; CA116(16): 158711f]

The preparation of microspheres of non-crosslinked PVA exhibiting goodcompressive strength by dispersing an aqueous solution of the polymer ina water-immiscible liquid, freezing the solution and crystallizing thePVA at a temperature of 0 to 10° C. is reported in Japanese patentpublication 62/45637, published on Feb. 22, 1987.

The preparation of semicrystalline hydrogels from copolymers derivedfrom vinyl trifluoroacetate and comonomers such as maleic acid isdescribed by R. Ofstead et al. in Adv. Chem. Ser., 223 (Polym. AqueousMedia) , 61-72 [CA112(6):42493z]

The preparation of reinforced composites by blending polyvinyl alcohol,water and alumina, compressing the resultant mixture and aging it withheating is described by Sakai et al. in Japanese patent publication no.1/236,274. An exemplified composite exhibited a flexural strength of 40MPa.

PVA/silica composite hydrogels are reported by S. Ikoma et al. in"Kobunshi Ronbunshu" 47(12), 1001-4 (1990) [CA114(8) : 63302t). Theimproved mechanical and theological properties achieved by the additionof silica are attributed to microscopic bond formation between thesilica and the PVA.

It is known in the art relating to reinforcement that for a reinforcingagent to function effectively it must be bonded to the matrix material.In the past this type of bonding has typically been achieved usingcoupling agents such as silanes containing organofunctional groups.

One objective of this invention is to improve the physical propertiessuch as tensile strength, modulus and toughness of non-crosslinkedpolyvinyl alcohol hydrogels by providing uniformly dispersed highlycrystalline regions that are bonded to the adjoining polymer matrix.

A second objective of this invention is to provide a method forintroducing highly crystalline regions into a polyvinyl alcoholhydrogel. The regions function as reinforcing agents for the hydrogel.

SUMMARY OF THE INVENTION

The objectives of this invention are achieved by novel reinforcedcomposite structures consisting essentially of a matrix and a dispersedphase, where the matrix is a hydrogel containing water and a solubilizedvinyl alcohol homopolymer, the dispersed phase contains a plurality ofuniformly distributed acicular regions consisting of fibrils formed froma highly oriented crystalline vinyl alcohol polymer, where the diameterof the said fibrils is less then 1 millimeter, the aspect ratio of thefibrils is from 2:1 to 1000:1, and where the composite is characterizedby a gradual transition in the degree of crystallinity at the interfacesbetween the matrix and the fibrils.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides reinforced composite structures consistingessentially of a matrix and a dispersed phase, where

the matrix is a hydrogel comprising water and a solubilized vinylalcohol homopolymer,

the dispersed phase comprises a plurality of uniformly distributedacicular regions consisting essentially of fibrils of an orientedcrystalline vinyl alcohol polymer wherein at least 50 percent of therepeating units are CH₂ CH(OH), the diameter of said fibrils is lessthan 1 millimeter, the aspect ratio of said fibrils is from 2:1 to1000:1, and

where said composite is characterized by a continuous transition indegree of crystallinity at the interfaces between said matrix and saidfibrils.

This invention also provides a method for preparing the presentreinforced hydrogels, said method comprising the sequential steps of

forming a solution of a vinyl alcohol polymer in a liquid consistingessentially of from 10 to 90 weight percent dimethyl sulfoxide, anyremaining portion of said liquid being selected from the groupconsisting of water, alcohols containing from 1 to 4 carbon atoms andmixtures of water and said alcohols;

blending the resultant solution with fibrils consisting essentially of acrystalline, oriented polyvinyl alcohol where said fibrils exhibit adiameter of less than 1 millimeter and an aspect ratio of from 2:1 to1000:1;

heating the resultant dispersion under conditions that disrupt thecrystallinity in the surface layer of the fibrils without disrupting thecrystalline structure in the remaining portion of the fibrils, and

cooling said dispersion under conditions that initiate formation of agel.

The characterizing feature of the present composites is a continuoustransition in degree of crystallinity at the interfaces between thesubstantially non-crystalline polyvinyl alcohol hydrogel matrix and theoriented crystalline acicular fibrils of vinyl alcohol polymer thatfunction as the reinforcing agent for the composite. For the purposes ofthis invention, the "continuous transition in the degree ofcrystallinity" between the matrix and the fibrils should be understoodto mean that as observed by optical microscope the discontinuouscharacter of the initial boundary between the fibrils and polymer matrixhas been substantially eliminated.

In a preferred embodiment the reinforcing agent is uniformly dispersedsegments of small diameter, highly crystalline fibers formed from avinyl alcohol polymer. The fiber segments are typically from 50 micronsto about 20 mm. in length and from 5 microns to about 0.5 mm. indiameter. The aspect ratio of the segments is typically from 2:1 to1000:1. No appreciable reinforcement is observed below these ranges.

Bonding between the fiber segments, also referred to as fibrils, and thematrix of vinyl alcohol polymer hydrogel is achieved by disrupting thecrystalline surface layer of the fibrils while they are dispersed in asolution of a vinyl alcohol polymer containing water and dimethylsulfoxide as the major components of the solvent. The resultantdispersion is subsequently cooled under conditions that will initiateformation of the hydrogel that forms the matrix of the presentcomposites.

The dispersion of the fibrils in the solubilized vinyl alcoholhomopolymer is heated at a temperature of from 50 to about 110 0 C.,preferably from 75° to 850 C., for a period of time sufficient todisrupt the crystalline structure at the surface of the fibrils withoutsubstantially disrupting their interior crystalline structure andorientation.

Disruption of the surface of the fibrils can be observed under anoptical microscope as the gradual disappearance of the initial clearlydefined boundary at the interface between the fiber and the solution ofvinyl alcohol polymer as the heating period progresses. A preferredmethod for achieving disruption of the fibrils' surface is described ina subsequent portion of this specification.

The amorphous region resulting from disruption of the crystallinestructure at the surface of the fibrils is identical in structure to thesolubilized vinyl alcohol homopolymer. This amorphous region serves asthe adhesive that bonds the fibrils to the PVA matrix when thedispersion is cooled to below about 10° C. but above the freezing pointof the polymer solution.

The Hydrogel- and Fiber-Forming Vinyl Alcohol Polymers

As used in this specification the term "vinyl alcohol homopolymer"refers to the polymer that forms the matrix of the present composites.

The vinyl alcohol polymer from which the fibrils of the presentcomposites are formed contain at least 50 mole percent of repeatingunits corresponding to the formula --CH₂ CH(OH)--.

The polymers used to prepare the matrix and the fibrils of the presentcomposite materials are in turn typically prepared by hydrolysis orsaponification of polymers containing repeating units derived from vinylacetate or other vinyl ester of a carboxylic acid.

The degree of hydrolysis of vinyl alcohol polymers varies depending upontheir intended end use. The vinyl alcohol polymers used to prepare boththe hydrogel matrix and reinforcing fibrils of the present compositesare preferably at least 98 percent hydrolyzed and are linear or containat most a minimal degree of branching. The reason for this preference isto achieve the maximum degree of intermolecular interaction such ashydrogen bonding.

The vinyl alcohol homopolymers that form the present hydrogels aretypically not crosslinked. Crosslinking can detract from physical andchemical properties of the hydrogel.

Depending upon the physical and chemical properties desired in the finalcomposite, the vinyl alcohol polymers used to prepare the fibrils thatserve as the reinforcing agents of the present composites can becrosslinked. Methods for crosslinking include the use of radiation orother means of generating free radicals and reaction of the hydroxylgroups of the polymer with aldehydes, borates or polyfunctional organiccompounds such as carboxylic acids.

The molecular weight of the polyvinyl alcohol used to prepare thepresent hydrogels and fibrils is also determined by the end use of thepolymer. Regardless of the intended end use, the polyvinyl alcoholshould have a weight average molecular weight of greater than 44,000,preferably at least 75,000. Commercially available polymers withmolecular weights of from 75,000 to 440,000 are preferred, particularlythose polymers containing relatively large concentrations ofsyndiotactic or isotactic segments within the polymer molecules.

Preparation of the Solubilized Vinyl Alcohol Polymer

The polymer solution into which the fibrils are dispersed is prepared byheating a vinyl alcohol homopolymer in the presence of a solvent mixturecontaining a mixture of dimethyl sulfoxide with water and/or an alcoholas the major, preferably the sole, components. The dimethyl sulfoxideconstitutes form 10 to 90 weight percent, preferably from 60 to 90weight percent, of the solvent.

The preferred polymer concentration range will depend upon the molecularweight of the polymer. Typically the properties of PVA hydrogels,particularly tensile strength and elongation at break, increase withincreasing concentration and/or molecular weight of the polymer. Polymerconcentrations of from about 5 to about 20 weight percent, based on thecombined weight of polymer and solvent, are preferred.

Solubilization of the polymer can be facilitated by heating thepolymer/solvent mixture to temperatures from 50° C. up to the boilingpoint of the mixed solvent and stirring until the polymer is completelydissolved. Heating is preferably done under an atmosphere of nitrogen orother inert gas to minimize degradation of the vinyl alcohol polymer.

The Polyvinyl Alcohol Fibers and Fibrils

Fibers can be prepared from solutions of vinyl alcohol polymers usingconventional spinning and drawing methods taught in the prior art,including U.S. Pat. Nos. 4,765,967 to Hyon et al. and 4,851,168 to D.Graiver et al., the relevant portions of which are incorporated byreference. A detailed discussion of the preparation and properties ofpolyvinyl alcohol fibers is contained in a text by I. Sakurada entitled"Polyvinyl Alcohol Fibers" (Marcel Dekker, New York, 1985).

Polymer solutions that are converted into fibers typically contain from2 to about 30 percent by weight of a polyvinyl alcohol or a vinylalcohol copolymer exhibiting a molecular weight of at least 66,000. Asdiscussed in the preceding section of this specification, the highestpossible molecular weight polymers are desirable when the objective isto maximize tensile properties of the final fiber.

The lowest concentration value for a given molecular weight polymer ispreferred, based on the ability of these compositions to form fibersexhibiting diameters of 50 microns or less with a minimum amount ofdrawing. These small diameter fibers are desirable based on theiruniformity and excellent tensile properties. Most preferably theconcentration of polymer in the spinning solution is from 2 to about 10weight percent.

The temperature of the liquid or gas into which the spinning solution isextruded and coagulated can range from just above the freezing point ofa liquid coagulating bath to ambient or higher for a gaseous coagulatingmedium.

The oriented crystalline structure that characterize the fibrils used toprepare the present composites is developed by drawing the extrudedfibers. The extent to which fibers are drawn is often referred to as thedraw ratio, which can be up to 10 or higher, depending upon the degreeof orientation desired.

Fibers suitable for use in preparing the reinforced hydrogels of thepresent invention are less than one millimeter in diameter, preferablyfrom 5 microns to about 0.5 mm.

Prior to being dispersed in the solubilized vinyl alcohol polymer usedto prepare the present compositions the fibers are cut into segments,also referred to in this specification as fibrils, measuring from 50microns to 2 cm. in length. The ratio of the length of these fibrils totheir diameter, referred to as their aspect ratio, is typically from 2:1to 1000:1.

Formation of the Fibril Reinforced Hydrogel

Precursors of the present reinforced hydrogels are prepared bydispersing from 0.1 to about 20 weight percent of fibrils, based on theweight of the solubilized polyvinyl alcohol, into the polymer solution.To avoid dissolving the fibrils at this stage of the process thedispersion is preferably prepared under ambient conditions where thetemperature of the polymer solution is from 20° to 30° C.

The critical Step of the present method involves heating the dispersionof fibrils for a period of time and at a temperature sufficient todisrupt the crystalline structure at the surface of the fibrils withoutmodifying their oriented crystalline interior portion. During this stepthe dispersion is stirred while being heated to a temperature of from50° to 110° C., preferably from 75° to about 85° C. Samples of thedispersion are withdrawn periodically and examined under an opticalmicroscope for evidence of a change in crystallinity of the fibrilsurface.

At the beginning of the heating period a clearly defined boundary isobserved at the interface between the highly oriented crystallinefibrils and the polymer solution. As heating of the dispersion iscontinued this boundary is transformed from a clearly defined one to acontinuous transition between the crystalline regions of the fibrils andthe relatively amorphous polymer solution. It is at this point thatheating and stirring of the dispersion are discontinued.

The heating period required to disrupt the crystallinity at only thesurface of the fibrils is determined by a number of variables, includingthe molecular weight and crystallinity of the vinyl alcohol polymer inthe fibrils and the efficiency of heat transfer between the polymersolution and the surface of the fibrils. Heating periods of from 10 to30 minutes at a temperature of 80° C. are typical. The dispersion ispreferably stirred to optimize the transfer of heat throughout thedispersion, prevent localized overheating and ensure a uniformdistribution of fibrils throughout the dispersion.

Continued heating beyond the point of initial liquefaction of thepolymer at the surface of the fibrils is not only unnecessary, it may bealso undesirable as it will gradually destroy their highly orientedcrystalline structure.

In accordance with the present method, the dispersion of fibrils in asolubilized polyvinyl alcohol is poured into a container of the desiredshape and cooled to a temperature below -10 degrees, preferably below-20 degrees C., for a period of time sufficient to convert thesolubilized vinyl alcohol polymer to a non-flowing gel.

The time period required to form a gel depends upon a number ofvariables, including the concentration and molecular weight of thepolymer and the raze at which the solution is cooled. Taking all ofthese variables into account, the time period during which the solutionis cooled is generally from 2 to 16 hours.

The present inventors found that for the types and concentrations ofpolymers and the range of processing conditions they evaluated nosignificant additional improvement in properties of the final hydrogelwas apparent after 16 hours of cooling.

To optimize the properties of the final fibril-reinforced hydrogel it isdesirable to replace the dimethyl sulfoxide portion of the polymersolvent with water following conversion of the polymer solution to agel. A preferred method for achieving this replacement is to allow thegel to warm to ambient temperature and then place it in a water-misciblealcohol such as methanol, ethanol or propanol that is miscible withdimethyl sulfoxide but is a non-solvent for the gel. The alcohol a moreeffective extractant for dimethyl sulfoxide than water, therebyconsiderably shortening the time required to replace all of thedimethylsulfoxide in the gel relative to prior art methods.

The time required to extract the dimethyl sulfoxide from the gel canvary from several minutes to several days. This time interval isdependent upon the surface to volume ratio of the particular sample.When this exchange has been completed the gel is removed from thealcohol bath and placed in a water bath to replace the alcohol. The useof a circulating water maintained at a temperature of from just abovefreezing to about 60 degrees C. is preferred.

The water content of the final hydrogel can range from 20 to about 98weight percent. The preferred range is determined by the end useapplication of the hydrogel.

Numerous end use applications for high strength, reinforced PVAhydrogels are disclosed in the prior art. These applications include butare not limited to fabrication into shaped articles such as films,tubing, fibers and contact lenses. The composites are also useful forincorporation into and biocompatible medical products such as cathetersand prostheses to replace damaged or missing limbs or appendages such asfingers and toes.

Hydrogels formed from vinyl alcohol polymers can also be used asvehicles for the controlled release of various materials, includingdrugs and other medicaments, air fresheners, perfumes, and biologicallyactive materials such as pesticides, fertilizers and herbicides.

EXAMPLE

The following example describes a preferred reinforced compositehydrogel of this invention and a method for preparing the hydrogel, andshould not be interpreted as limiting the scope of the invention asdefined in the accompanying claims. Unless specified all parts andpercentages in the example are by weight and viscosities were measuredat 25° C.

A glass reactor equipped with a water cooled reflux condenser,mechanically operated stirrer and nitrogen inlet was charged with 30.26parts of distilled water, 121.04 parts of dimethyl sulfoxide and 17parts of finely divided polyvinyl alcohol exhibiting a weight averagemolecular weight of 86,000. The stirrer was operated to achieve ahomogeneous dispersion of the solid polymer, at which time thedispersion was heated to 100° C. for a period of time sufficient tocompletely dissolve the polymer.

The resultant solution of polyvinyl alcohol was then allowed to cool to80° C., at which time 1.7 parts of polyvinyl alcohol fiber segmentsmeasuring 1 mm. in length and 16 microns in diameter were dispersed intothe solution. The fibers were cut from a tow supplied by Unitika K.K.The fibers were wet spun and the degree of polymerization of the polymerwas 1700.

Samples were removed from the heated dispersion at intervals of about 10minutes and a drop placed on a microscope slide. The slide was thenexamined under an optical microscope. Heating and stirring werediscontinued as soon as the initial sharply defined boundary at theinterface between the crystalline fiber segments (fibrils) and thesolubilized polyvinyl alcohol became indistinct. The dispersion was thenpoured as a 0.13 cm-deep layer into a Petri dish and the dish was placedin a freezer maintained at a temperature of -20° C. for 16 hoursfollowed by 4 hours at 4° C. The gel was removed from the mold andplaced in a bath of methanol to extract the dimethyl sulfoxide. Themethanol was then replaced with water by immersing the gel for 12 hoursin a circulating water bath maintained at ambient temperature.

The tensile strength and elongation of the reinforced hydrogel weredetermined using a standard tensile bar that had been cut from the gel.Tear strength was determined as described in ASTM test procedure D 624(Die B Tear Strength) using a tensile bar with a 1/4 inch (0.6 cm)-longnotch. These property values were dependent upon the length of theheating period of the dispersion of fibrils in the PVA solution. Thesevalues are recorded in the following table as "tensile", "elongation"and "tear" together with the values for a control that did not containany fibrils.

                  TABLE 1                                                         ______________________________________                                        Heating Time                                                                            Tensile     Elongation                                                                              Tear Strength                                 (Min)     (psi/Mpa)   (%)       (pli//kN/m)                                   ______________________________________                                        20        242/1.7     255       21//3.8                                       40        367/2.5     336       19//3.4                                       180       337/2.3     285       23//4.0                                        0*       146/1.0     213       15//2.7                                       ______________________________________                                         * = Control Example                                                      

That which is claimed is:
 1. A method for preparing a reinforcedpolyvinyl alcohol composite, said method comprising the steps of1)forming a solution of a vinyl alcohol homopolymer in a liquid consistingessentially of from 10 to 90 weight percent dimethyl sulfoxide, anyremaining portion of said liquid being selected from the groupconsisting of water, alcohols containing from 1 to 4 carbon atoms andmixtures of water and said alcohols; 2) forming a dispersion by blendingthe resultant solution with from 0.1 to 20 weight percent, based on theweight of said homopolymer, of fibrils consisting essentially of acrystalline, oriented polyvinyl alcohol, where said fibrils exhibit adiameter of less than 1 millimeter and an aspect ratio of from 2:1 to1000:1; 3) heating the resultant dispersion to a temperature of from 50to 110 degrees celsius and stirring the dispersion so to disrupt thecrystallinity of the surface layer of said fibrils without disruptingthe crystalline structure in the remaining portion of said fibrils, 4)cooling the dispersion sufficiently to initiate formation of a gel, and5) replacing substantially all of the non-aqueous liquids in said gelwith water to obtain a composite hydrogel containing from 20 to 98weight percent water.
 2. A method according to claim 1 where the weightratio of the fibrils to said homopolymer is from 0.1 to 20, the lengthof said fibrils is from 50 microns to 20 mm, the dimethyl sulfoxideconstitutes from 60 to 90 weight percent of said liquid, theconcentration of solubilized vinyl alcohol polymer is from 5 to 20weight percent, based on the combined weight of solubilized polymer andliquid, the dispersion is heated to between 50° and 110° C. and is thencooled to below -10° C.
 3. A method according to claim 2 where the vinylalcohol polymer present in the fibrils is a homopolymer, said dispersionis heated to between 75° and 85° C. and cooled to below -20° C.